1. Field of the Invention
The present invention relates to a method for restoring sterility of a male sterile plant, such as a plant of the family Poaceae, particularly, a plant of the subfamily Pooideae, and to a composition for restoring sterility of a plant of the subfamily Pooideae. In particular, the present invention relates to a method and a composition for restoring sterility of a plant which becomes male sterile due to a high-temperature or low-temperature stress. Examples of the plant include plants of the family Poaceae, particularly, plants of the subfamily Pooideae.
2. Description of the Related Art
In the cases of plants of the subfamily Pooideae of the family Poaceae such as wheat and barley, temperature rise due to global warming or unusual weather causes formation insufficiency in the pollen formation process, which leads to worldwide decrease in crop production. Meanwhile, in the case of rice plant (Oryza sativa), as is known as the chilling injury in the Tohoku region, unusual low-temperature due to “Yamase,” which is said to occur every approximately 10 years, causes formation insufficiency in the pollen formation process, and greatly lowers the yield.
In the low-temperature injury (chilling injury) of rice plant in the Tohoku region, anther wall tapetal cells become hypertrophic to inhibit pollen formation when the rice plant is exposed to such a low-temperature stress that the highest temperature is less than 20° C. for several days at the booting stage (immediately after the meiosis of pollen mother cells). The anther wall tapetal cells are cells that supply an influence to pollen and are destined to eventually collapse due to apoptosis.
Meanwhile, in contrast to rice plant, plants of the subfamily Pooideae such as wheat and barley are highly sensitive to a high-temperature stress. Cell division of anther wall cells and pollen mother cells stops under high-temperature conditions of 30° C. during the day and 25° C. at night (particularly, under a condition of 25° C. or above at night). If the high-temperature conditions continue for four days or longer, these plants turn into a completely non-restorable state. Anther wall tapetal cells collapse at an early stage. As a result, normal pollen cannot be formed, which results in pollen sterility (male sterility). This pollen sterility eventually leads to reduction in seed fertility. The stop of cell division and the collapse at an early stage are observed only in anthers. Such phenomena are observed only in male pollen formation, without affecting the growth of pistils, leaves, stems, and the like (Sakata et al., Journal of Plant Research (2000) 113, 395-402, and Abiko et al., Sexual Plant Reproduction (2005) 18, 91-100). An exhaustive expression analysis using a DNA microarray showed that large scale changes in gene expression occurred during the high-temperature injury. The expression of auxin repressed protein genes, whose expression is repressed by an auxin, is induced in high-temperature injury of young panicles (Oshino et al., Molecular Genetics and Genomics (2007) 278, 31-42). In other words, studies were made on the possibility that the expression of an auxin, which is one of the plant hormones and plays an important role in division, growth, development, and differentiation of cells, is reduced under a high-temperature condition in an anther-specific manner. Particularly, it is known that when the plant is placed in a high-temperature environment as described above at the start of the five-leaf stage, tapetal cells and pollen mother cells stop their development and differentiation and produce no pollen in anthers formed subsequently (Oshino et al., Molecular Genetics and Genomics (2007) 278, 31-42).
To solve these problems development is under way for cultivars which exhibit enhanced resistance to a high-temperature or low-temperature stress, by conventional breeding or by constructing recombinants through recombination technologies, for example.
An auxin is a generic term for plant hormones which promote mainly the growth of plants. Naturally occurring auxins and synthesis auxins are known. Naturally occurring auxins include indole-3-acetic acid (IAA) and indolebutyric acid (IBA). Synthesis auxins include 4-chloroindoleacetic acid, phenylacetic acid, 2,4-dichlorophenoxyacetic acid (2,4-D), α-naphthaleneacetic acid (NAA), 2,6-dichlorobenzoic acid, indolebutyric acid (IBA), 4-chlorophenoxyacetic acid, ethyl 5-chloroindazoleacetate, naphthoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and the like. These auxins have been used as growth control agents. Under laboratory environments, auxins are used for tissue culturing and the like. In farm fields, 2,4-D and the like are used as herbicides.